REVISITING SUPERBOLT'S SUPER TORQUENUT: Since our last visit, many more compressor related applications of the torquenut have been uncovered

Compressor Tech Two // September 2004 by Joe Kane

Applications of Superbolt's Torquenut are limited only by one's imagination - or need, according to Allan Steinbock, vice president, for the company based in Carnegie, Pennsylvania, U. S. A. "We've always considered the Torquenut a solution to problems related to high-torque requirements for large bolting systems and wide variations in temperature and pressure," he said. "Industries using large numbers of compressors are ideally suited for Torquenut applications."

Steinbock continues, "All Superbolt products are based on our patented multi-jackbolt design which provides superior tightening while requiring only a hand torque wrench. This eliminates expensive, unsafe and inaccurate bolting practices. Reduced downtime is accomplished becuase our products are designed not to vibrate loose on a properly designed joint. Installation time can be reduced and removal becomes a simple task. As a consequence, maintenance associated with large fasteners is greatly simplified."

The whole idea behind bolting is to join two or more machinery components with an initial loading that will be greater than the operating load to which the joint will be subjected. The key is achieving the necessary preload accurately, quickly, safely and in a cost-effective way. The Superbolt Torquenut, or Multi-Jackbolt Tensioner (MJT), addresses these bolting issues. These tensioners are designed as direct replacements for hex nuts or standard OEM nuts. They can be threaded onto a new or existing bolt, stud, threaded rod or shaft. Typical reciprocating compressor applications include crosshead jamnuts, attaching distance pieces, piston end nuts for attaching pistons to piston rods, connecting rods, anchor bolts, couplings and piping. They can be used anywhere that a large hex nut or standard nut is needed; and with greater security under fluctuating loads, temperature variations and extreme vibration.

The same concept applies to joining two halves of a casing for centrifugal compressors as well as joining two halves of a flexible disc coupling. Applications such as these require a high degree of torque accuracy which is easy to achieve with MJTs. A relatively low level of torque exerted on each jackbolt in the Torquenut can achieve a very high equivalent hex-nut torque, resulting in a high mechanical advantage. For example, one would need 18,925 lb.ft. (25,681 Nm) of torque to stress a 3-in. (76mm) stud to 428,400 lb. (1945 kN) using a hex nut. With a 3-in. MTX Multi-Jackbolt Tensioner, only 114 lb.ft. (155 Nm) on each of the jackbolts is needed to produce the same bolt load.

The Superbolt MJT consists of a nut body that threads onto a stud or rod, a series of jackbolts arranged around and threaded through the nut body and a hardened washer or bearing surface. The hardened washer is placed over the stud or rod first and then the tensioner is threaded onto the rod for positioning. With a simple hand torque wrench, the jackbolts are tightened uniformly until the proper preload is achieved. The tensioner flexes, which adds elasticity to the system, helping to accommodate fluctuating loads and temperatures that could tend to loosen the joint. The preload is achieved without thread welding or galling, which is a common problem with large ordinary nuts being torqued on studs. The amount of torque required for adequate preload with jackbolt tensioners is a fraction of that needed for ordinary hex nuts. This accounts for the simple tooling required to install the stud tensioner.

The Superbolt MJT can maintain sufficient preload when length changes occur in bolting systems because of thermal expansion or the settling of gaskets in gasketed joints. That is because there is added elasticity inherent in the MJT. It can prevent joints from leaking or seal leaking joints. Tensioners are available in sizes ranging from 3/4 to 32 in. (19 to 813 mm) and in load capacities ranging from 10,000 lb. to 20 million lb. (45.4 kN to 90.8 x 106 N). Total load tensioning of some 10,000 tons (9072 tonnes) can be achieved with a 3/4 in. wrench. Superbolt tensioners are available for high-temperature applications in ASTM A193-B7, B16, B8M and ASTM A453 Grade 660 alloys as well as Inconel and other "superalloys".